There have been proposed a hyper wide-angle zoom lens suitable for single-lens reflex cameras, such as JP-A (Japanese Preliminary Publication of Unexamined Patent Application) No. 2006-039531. This hyper wide-angle zoom lens has four groups of lens pieces, namely, the leading or foremost 1st lens group of negative refractive power, the succeeding 2nd lens group of positive refractive power, the 3rd lens group of negative refractive power, and the trailing or rearmost 4th lens group of positive refractive power arranged in this order from the closest to an objective field toward an imaging plane; and such zoom lenses have their respective 1st and 2nd lens groups come closer to each other, their respective 2nd and 3rd lens groups come farther apart from each other, and their respective 3rd and 4th lens groups also come closer during zooming from the wide-angle end to the telephoto end, meeting requirements expressed in the following formula:2.9<bfw/fw<5.03.1<f4/fw<4.50.1<fw/f2<0.42where bfw denotes a back focus at the wide-angle end, fw is a focal length of the entire optics at the wide-angle end, and f2 and f4 are respectively the focal length of the 2nd and 4th lens groups.
The above-mentioned prior art wide-angle zoom lens is all fourfold optics zoom lenses, including the lens groups of negative-positive-negative-positive refractive power combined in this order. They are intended to implement more than 100 degrees of angle of filed at the wide-angle end and to attain high zoom ratio more than 2 as well.
Specifically, the zoom lens disclosed in JP-A-2006-039531 is directed to the angle of filed as wide as 105.8 degrees at the wide-angle end and the zoom ratio as high as 1.05 to 2.36.
This prior art wide-angle zoom lenses typically have their respective 1st lens groups shaped aspherical on more than one major surfaces of the lens pieces. Especially, the zoom lens as set forth in Patent Document 1 comprises the front-end lens piece shaped in concave surface, facing toward the imaging plane, so as to be a meniscus lens of very strong negative refractivity. Such lens optics has a front surface of the front-end lens piece shaped aspherical, thereby downsizing the 1st lens group, as a whole, and compensating for various types of aberration caused therein.
The zoom lens disclosed in Patent Document 1 has its front-end lens piece closest to the objective field deliberately imparted enhanced negative power and has its machinable glass surface shaped aspherical so as to provide the angle of field as wide as 105 degrees at the wide-angle end, thereby advantageously retaining an effective aperture of a forward lens frame as small as possible. However, it is hard to provide the further wider angle of field at the wide-angle end.